[MR2] Handle transfer to LEA device during broadcast

Set fallback to target route's LEA group id so that BT can set the
device active after transitting to unicast

Bug: 438156758
Test: atest AudioManagerRouteControllerTest
Flag: com.android.media.flags.enable_output_switcher_personal_audio_sharing
Change-Id: I136d005b08806c5d3e41c4aa73bf45d7db463225